The present invention relates to a finely divided silica with high structure, a method of preparing the silica and the use of the silica as a delustering agent in films of the paint, lacquer, varnish and enamel type.
It is known that the delustering capacity of a silica depends on various factors such as e.g. the type of silica, the particle size, the refraction index and also on the vehicle system. Particle form and particle-size distribution of the secondary particles as well as the total effective specific particle volume are of particular significance.
A number of other requirements are placed on delustering silicas in addition to high efficiency, expressed by the reduction of the degree of glossiness in comparison to a non-matte paint film. Thus, for example, the drying behavior of the film coatings should not be adversely affected, no excessive thickening of the paint coating system should occur and the scratch resistance of the film coatings should not be lowered by the silica added. The behavior in suspension of the silica is an important point. The tendency of silica to settle and to form hard sediment which can only be stirred up again with difficulty can be prevented or at least improved by a number of measures. Thus, for example, DE-AS 15 92 865 describes the impregnation of silica during the preparation process by means of wax emulsions.
The addition of synthetic silica as delustering agent is known. See for example the publications:
EPO-Patent 0,008,613 PA1 German Patent 24 14 478 PA1 German DAS 17 67 332 PA1 German OLS 16 69 123 PA1 German DAS 15 92 865. PA1 7.5 parts by weight paint prepaste Tack.RTM. 1 PA1 47 parts by weight fatty-acid modified alkyd resin 60% in xylene (Alftalat AR 481 m) PA1 24 parts by weight melamine resin Maprenal MF 800 55% in isobutanol PA1 6 parts by weight butanol PA1 10.5 parts by weight xylene PA1 1 part by weight silicon oil Baysilon paint additive OL 17 1% in xylene.
Numerous methods of preparing synthetic silica are also known (cf. Ferch in "Chem.-Ing.-Techn." 48, pp. 922-33 (1976)).
According to German Patent 24 14 478, aerogel-like delustering agents can be obtained by the structuring of pyrogenically prepared silica. To this end, pyrogenic silica is moistened with alkaline adjusted water, ground and dried. This procedure of preparing delustering agents is complicated and expensive.
The delustering agent according to German Patent 24 14 478 is an excellent delustering agent. However, it has the disadvantage that it thickens the film compositions too heavily. A solvent must therefore be added in addition for working with a spray gun to facilitate application of the films to a surface to be coated.
A wax-coated precipitated silica acid is prepared according to the process described in DE-PS 15 92 865 and is used as delustering agent. This delustering agent has the disadvantage, however, that the paints prepared with it exhibit an undesirable blue bloom on dark surfaces.